Device for the achromatic magnetic deflection of a beam of charged particles and an irradiation apparatus using such a device

ABSTRACT

An achromatic magnetic deflection device for deflecting by an angle φ between π and 2π a beam of charged accelerated particles having different momentum. This device comprises an electromagnet provided with pole pieces delimiting three contiguous sectors, the whole of these sectors, having an axis of symmetry XX, presenting flat input E and output S faces and common faces F 1  and F 2  in an arc of a circle, the position, the radius of curvature of these faces F 1 , F 2  as well as the value of the magnetic induction in the sectors being chosen so that the different paths are substantially orthogonal both to faces F 1 , F 2  and to axis XX.

BACKGROUND OF THE INVENTION

The present invention relates to an achromatic magnetic deflection device for deflecting by an angle φ a beam of charged accelerated particles (electrons for example), these particles being able to present a large range of moments of quantities of movement.

The deflection device of the invention enables in particular a beam of electrons accelerated between 10 and 20 Mev for example to be deflected by an angle φ>π, without having to modify the values of the magnetic fields created in the air gaps of the pole pieces forming part of the deflection device.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a device for the achromatic magnetic deflection of a beam of accelerated charged particles comprising at least one electromagnet having pole pieces delimiting air gaps in which are created magnetic fields having the same direction and specific values so that the paths of the particles have the form of loops whose lengths depend on the momentum of the particles, these pole pieces delimiting a first, a second and a third magnetic sector disposed one after the other and joined together, the whole of these magnetic sectors having a plane of symmetry perpendicular to the plane of the mean path of the beam of particles and intersecting this plane along an axis XX, the magnetic deflection device presenting successively to the beam of particles a flat input face, a first curved face, a second curved face and a flat output face, the input and output flat faces forming therebetween an angle 2α, the first and second curved faces, as well as the axis of symmetry XX, being substantially orthogonal to the different paths of the particles, the values of the magnetic inductions created in the first and third magnetic sectors being respectively equal to KB_(o), B_(o) being the value of the magnetic induction in the second magnetic sector and K a numeric coefficient less than 1.

The above and other objects, features and advantages of the present invention will become apparent from the following description, given solely by way of non-limiting illustration, when taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of a magnetic deflection device according to the invention.

FIG. 2 shows the paths of the particles in the embodiment of FIG. 1.

FIG. 3 shows a second embodiment of a magnetic deflection device according to the invention.

FIG. 4 shows the paths of the particles in the device of FIG. 3.

FIGS. 5 and 6 show respectively the variation of the ratio K of the radii of curvature of the different paths for the embodiments shown in FIGS. 1 and 3.

FIGS. 7 and 8 show respectively a top view and a sectional view along the axis of symmetry XX of a pair of pole pieces used in the device of the invention.

FIG. 9 shows the lens effect obtained in the horizontal plane with the devices of FIGS. 2 and 3.

FIGS. 10 and 11 show respectively a variation of the device of the invention and the lens effects of this device on the beam, in the horizontal and vertical planes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The achromatic magnetic deflection device according to the invention, such as shown in FIG. 1, for deflecting by 270° a beam of charged particles, in particular electrons, is formed by an electromagnet having magnetic coils (not visible in the figure) and comprising a pair of pole pieces A, A (only one pole piece A is visible in the figure) of such a shape that they delimit three magnetic sectors M₁, M₂, M₃ having a plane of symmetry perpendicular to the plane in which move the mean paths of the beam of particles and intersecting this plane along an axis XX inclined by an angle α=π/4 in relation to the mean path of the incident beam f_(i). The magnetic sector M₁ is delimited by a flat input face E and a face F₁ substantially circular in shape, with a radius of curvature R, the magnetic sector M₃ is delimited by a flat output face S and a face F₂ identical to face F₁ and the intermediate contiguous magnetic sector M₂ being delimited by faces F₁ and F₂. The input E and output S faces form therebetween an angle 2α=π/2.

The heights of the air gaps of magnetic sectors M₁ and M₃ on the one hand, and M₂ on the other are such that the values of the magnetic inductions created respectively in these magnetic sectors M₁, M₂, M₃ are equal to Bo/2, Bo, Bo/2, the particles then being deflected by an angle θ in each of the magnetic sectors M₁ and M₃ and by an angle 2 φ in magnetic sector M₂, the sum 2θ+2φ of these angles being equal to 2π-2α=3π/4 (FIG. 2).

FIG. 2 shows the paths t₁, t₂ and t₃ of the particles having respectively an energy E₁, E₂, E₃. The path t₁ has a center of curvature C₁ in magnetic sector M₁ and a center of curvature C₂ in magnetic sector M₂. This path t₁ is orthogonal on the one hand to faces F₁, F₂ and to the axis of symmetry XX of the device. In the magnetic deflection device of the invention, the center of curvature C₂ of the paths in magnetic sector M₂ must be situated on the axis of symmetry XX of the deflection device. This center of curvature C₂ of the paths may be defined, in an orthonormed plane xy, such as shown in FIG. 2, by the relationships:

    x.sub.C.sbsb.2 =(r.sub.1 -r.sub.2) sin θ             (1)

    y.sub.C.sbsb.2 =r.sub.1 (1-cos θ)+r.sub.2 cos θ

r₁ being the radius of curvature of the paths in magnetic sector M₁ (and in magnetic sector M₃ not shown) and r₂ being the radius of curvature of the paths in magnetic sector M₂.

So that the centers of curvature C₂ are situated on the axis of symmetry XX, the following relationship must be verified: ##EQU1##

If we assume: (r₂ /r₁)=K, the relationship (3) becomes ##EQU2## but: ##EQU3## R being the radius of curvature of faces F₁, F₂.

The value of K is then given by the relationship: ##EQU4##

FIG. 5 shows the variation of K as a function of θ, for α=45°, (b/R)=0.5. It is to be noted that K is substantially equal to 0.5 for values of θ between 75° and 100°, which corresponds to an energy range between 1.4 E₀ and 0.8 E₀.

FIG. 3 shows another embodiment of a magnetic deflection device in accordance with the invention for deflecting the incident beam f_(i) by an angle 2α equal to 240°, this deflection being achromatic. This magnetic deflection device comprises an electromagnet having magnetic coils (not shown) and provided with a pair of pole pieces of a shape and dimensions such that they delimit three contiguous magnetic sectors M₁₀, M₂₀, M₃₀. Magnetic sector M₁₀ presents to the beam a flat input face E and a face S₁₀ having the form of an arc of a circle with radius R₁₀, magnetic sector M₃₀ has a flat output face S and a face S₂₀ identical to face S₁₀, whereas magnetic sector M₂₀ contiguous to magnetic sectors M₁₀, M₃₀ is delimited by faces S₁₀ and S₂₀. The heights of the air gaps of magnetic sectors M₁₀, M₂₀ and M₃₀ are such that the magnetic inductions created in each of these sectors are respectively equal to KB_(o), B_(o) and KB_(o).

FIG. 4 shows in detail the different paths of the particles having different momentum in the deflection device shown in FIG. 3. In this embodiment, the ratio b/R has been chosen equal to 0.63, b being the distance separating the mean path of incident beam f_(i) from point I, the intersection of axis XX with the input face E of the deflection device. For the different paths shown t₁₀, t₂₀, t₃₀, t₄₀, the centers of curvature C₂ in magnetic sector M₂₀ are substantially placed on the axis of symmetry XX. These different paths t₁₀, t₂₀ . . . correspond to energy particles respectively equal to E₁₀, E₂₀, E₃₀, E₄₀.

FIG. 6 shows the variations of K=(r₂ /r₁) as a function of θ. It may be noted that, in this embodiment (FIG. 3) K is substantially equal to 0.36 for values θ between 55° and 100° and the magnetic inductions created in the air gaps of magnetic sectors M₁₀, M₂₀ and M₃₀ are respectively equal to 0.36 B_(o), B_(o) and 0.36 B_(o).

In the embodiments shown in FIGS. 1 and 3, the differences in value of the magnetic inductions in sectors M₁, M₃ and sector M₂ have been obtained with different heights of the air gaps of these magnetic sectors.

In FIG. 7 there is shown an embodiment of a pole piece A₁ in accordance with the invention and the magnetic coil which is associated therewith. Pole piece A₁, circular in shape, is formed by an element a₁ (FIG. 8) made from magnetic material, soft iron for example, whose dimensions are defined by the operating characteristics of the deflection device (type of particles, energy of these latter, value of the magnetic inductions used), and by an element c₁ superimposed on element a₁ and fixed to this latter by means of three screws v₁, v₂, v₃ for example, this element c₁ delimiting the intermediate magnetic sector M₂ (or M₂₀). The thicknesses of elements a₁ and c₁ are chosen with respect to the value of the magnetic inductions used in magnetic sectors M₁, M₂, M₃ (or M₁₀, M₂₀, M₃₀) so as to avoid any saturation of the magnetic material forming pole piece A₁. An annular magnetic coil b₁ is disposed on pole piece A₁. Opposite pole piece A₁ is placed an identical pole piece A₂, associated with the annular magnetic coil b₂ identical to b₁ (FIG. 8).

In operation, in the embodiments shown in FIGS. 1 and 3 of the device of the invention, the different paths of the particles converge in the horizontal plane H in a focus F_(H) situated on the output face S of the third magnetic sector M₃ (FIG. 9) whereas in the vertical plane V, the whole of the deflection device behaves like a creeping space. If it is desired to obtain a stigmatic magnetic deflection system, i.e. for forming a pinpoint image of an object point situated outside the axis of the incident beam f_(i), the divergences of the beam must be compensated for both in the vertical plane V and the horizontal plane H. For that, it is sufficient for the mean path of incident beam f_(i) to form with the input face E of the magnetic deflection device an angle a little different from π/2 (FIG. 10).

FIG. 9 shows the lens effects obtained with a magnetic deflection device whose input E and output S faces are orthogonal to the mean path of the beam of particles.

FIG. 11 shows the action of the magnetic lenses formed by the magnetic deflection device of the invention, shown in FIG. 10, when this deflection device presents to the beam an input face E forming an angle a little different from π/2 with the mean path of this incident beam f_(i). In this case, the beam f_(i) is subjected to focusing both in the horizontal plane H and in the vertical plane V, this double focusing being situated at a distance l from the output face S of the deflection device, this distance l corresponding for example to the distance separating the output face S of the deflecton device and a target Q intended to be bombarded by a substantially pinpoint beam.

The examples given are not limiting. In particular, the construction of the intermediate magnetic sector M₂ (or M₂₀) may be different from the examples given. It may in particular form a separate element which will be joined to the end sectors M₁, M₃ (or M₁₀, M₃₀).

The magnetic deflection device of the invention presents several advantages. It is compact and simple to construct. Furthermore, it has a wide passband. It may be advantageously used in radiotherapy apparatus, doing away with the need to adjust the magnetic field for a wide energy range of particles.

It is apparent that within the scope of the invention, modifications and different arrangements can be made other than are here disclosed. The present disclosure is merely illustrative with the invention comprehending all variations thereof. 

What I claim is:
 1. A device for the achromatic magnetic deflection of a beam of charged accelerated particles, comprising at least one electromagnet having pole pieces delimiting air gaps in which are created magnetic fields having the same direction and specific values so as to obtain paths of particles having the form of loops whose lengths are a function of the momentum of the particles, said pole pieces forming a first, a second and a third magnetic sector disposed one after the other and joined together, the whole of these magnetic sectors having a plane of symmetry perpendicular to the plane of the mean path of said beam of particles, and intersecting this plane along an axis XX, forming an axis of symmetry, and magnetic deflection device presenting successively to the beam of particles a flat input face E, a first curved face F₁, a second curved face F₂ and a flat output face S, said input E and output S faces forming therebetween an angle 2α, said identical curved faces F₁ and F₂ as well as said axis of symmetry XX being substantially orthogonal to the different paths of said particles, the values of said magnetic inductions created in the first and third magnetic sectors being respectively equal to KB_(o), B_(o) being the value of the magnetic induction in the second magnetic sector and K a numerical co-efficient less than
 1. 2. A magnetic deflection device as claimed in claim 1, wherein the radius of curvature r₁ of the paths of the particles in the first and third magnetic sectors and the radius of curvature r₂ of the paths in the second intermediate magnetic sector are bound by the relationship: ##EQU5## r₁ and r₂ depending, for specific magnetic induction values in the different magnetic sectors, on the momentum of said particles,b being the distance separating the mean incident path from the intersection point I of input face E with the axis of symmetry XX of the magnetic deflection device, θ being the total angle of deflection of the particles in the first and third magnetic sectors, this angle θ depending on the momentum of said particles, for specific values of the magnetic induction, R being the radius of curvature of curved faces F₁ and F₂ respectively common to the first and second magnetic sectors, and to the second and third magnetic sectors, and wherein the deflection angle 2φ of the particles in the second magnetic sector is equal to 2[π-(α+θ)].
 3. A magnetic deflection device as claimed in claim 2, wherein a pair of pole pieces is provided whose form and dimensions are such that they delimit three successive contiguous magnetic sectors M₁, M₂, M₃, in which are respectively created magnetic inductions of value (B_(o) /2), B_(o), (B_(o) /2), the ratio K=(r₂ /r₁) being substantially equal to 0.5, wherein angle 2α is substantially equal to (π/2), and wherein the radius of curvature R of the intermediate faces F₁, F₂ is substantially equal to 2b.
 4. A magnetic deflection device as claimed in claim 3, wherein the air gap of the magnetic sector M₂ has a height equal to half of the height of magnetic sectors M₁ and M₃.
 5. A magnetic deflection device as claimed in claim 3, wherein the angles θ corresponding to the different paths are between 60° and 110°.
 6. A magnetic deflection device as claimed in claim 2, wherein a pair of pole pieces are provided whose form and dimensions are such that they delimit three successive contiguous magnetic sectors M₁₀, M₂₀, M₃₀, in which are created respectively magnetic fields of values substantially equal to 0.36 B_(o), B_(o), 0.36 B_(o), the ratio (r₂ /r₁) of the radius of curvature r₂ of the paths in magnetic sector M₂₀ and of the radius of curvature r₁ of the paths in magnetic sectors M₁₀ and M₃₀ being substantially equal to 0.36, said angle α being substantially equal to π/3, and said radius of curvature R₁₀ of the intermediate faces F₁₀, F₂₀ being substantially equal to 1.58 b.
 7. A magnetic deflection device as claimed in claim 6, wherein the height of the air gap of magnetic sector M₂₀ is substantially equal to a third of the height of magnetic sectors M₁₀ and M₃₀.
 8. A magnetic deflection device as claimed in claim 6, wherein the angles of rotation θ of the particles of different energies in magnetic sectors M₁₀ and M₃₀ are between 55° and 100°.
 9. A magnetic deflection device as claimed in 3, wherein each of the pole pieces is formed by a first element a₁, made from a magnetic material on which is fixed a second element c placed so as to reduce the air gap of the pole pieces corresponding to said intermediat magnetic sector and having the form of the intermediate magnetic sector.
 10. A magnetic deflection device as claimed in claim 6, wherein each of the pole pieces is formed by a first element a₁ made from a magnetic material on which is fixed a second element c placed so as to reduce the air gap of the pole pieces corresponding to the intermediate magnetic sector and having the form of said intermediate magnetic sector. 